1. Field of the Invention
This invention relates to circuit interrupters, and more particularly to air circuit interrupters.
2. Description of the Prior Art
Air circuit interrupters include a first type having no insulating housing at all and a second type having a molded insulating housing enclosing the entire interrupter mechanism, including its contact structures and operating mechanisms.
The first type of interrupter, having no housing, generally consists of circuit breakers for relatively large rated current capacity and has the advantages that inspection and maintenance are easy and that it does not require an expensive molded case. However, the dielectric strength between the current carrying portion and the operating mechanism is relatively low, and when an electric arc is established between the separated contacts, a leakage current may flow to the operating mechanism. Furthermore, since this type of circuit interrupter does not include a housing, the gas pressure at the arcing region, produced by the heavy current, does not increase sufficiently which causes insufficient gas flow to the arc extinguisher means, as compared to circuit interrupters with an insulating housing. For these reasons, the first type of circuit interrupters, having no housing, is not suitable for handling a large interrupting current within a small volume.
The second type of circuit interrupter has its current carrying portion and operating mechanism enclosed within an insulating housing, so that the pressure within the housing increases to a certain extent upon contact separation, which contributes somewhat to enabling an effective gas glow and a better interrupting performance. Therefore the rated interrupting capability of the second type of interrupter is usually high to a certain extent than that of the first type of interrupter. However, since the entire mechanism, including the contact structure and the operating mechanism, is housed in a single housing, the housing is inevitably relatively large, and it is still insufficient to achieve the desirable gas flow for arc extinction since the arc gas produced around the contact region still tends to spread partially against the arc extinguisher means. Moreover, since the housing is inevitably relatively large and expensive, it tends to be designed within as small dimensions as possible, which reduces the robustness of the operating mechanisms and the contacting force and also causes a reduction of the capability to carry and withstand high fault currents, i.e. so called rated short time withstand current. Furthermore, the housing of this type of circuit interrupter is expensive and causes a problem in the inspection and maintenance of the interrupter contacts since the total mechanism is housed in a combination of a base and a cover.
There is another means for providing higher interrupting capacity for both the first and second types of interrupters. According to this means, a repulsive magnetic force between the movable contacts, produced by a heavy short circuit current, is utilized to obtain a quick separation of the contacts, making possible a high interrupting ability. However this type of circuit interrupter lacks the short time current withstand ability and is inadequate for use in system selectivity co-ordination.
In the case of any of the circuit interrupter types, it is necessary to obtain a high pressure arc at the contact area and to exhaust the arc gas through the arc extinguisher means to outside the opening of the arc chamber as quickly as possible in order to make possible a high interruption performance. Since the arc speed is generally limited by the existence of arc extinguisher means which act as a fluid dynamic resistance against the arc flow, the high pressure and smooth movement of the arc are highly effective in obtaining a high arc speed.